Ethylene copolymers with a novel composition distribution and processes for making the same

ABSTRACT

A Ziegler-Natta catalyzed ethylene copolymer having a novel composition distribution in which comonomers are incorporated into the high molecular weight polymer molecules and distributed evenly among the entire polyethylene chains, and a method for making the same are provided. The resins having a novel composition distribution have controlled molecular weight distribution which is narrower than conventional ZN-ethylene copolymers but broader than single-site catalyzed ethylene copolymers. The resins having a novel composition distribution exhibit a superior tear strength and impact strength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to Ziegler-Natta (ZN) catalyzedethylene-alpha olefin copolymers having densities of about 0.870 g/cc orhigher, processes for making the same, and articles made of this newcomposition.

2. Description of the Related Art

Various types of polyethylene are known in the art. Low densitypolyethylene (LDPE) is generally prepared at high pressure using freeradical initiators and typically has a density in the range of0.9100-0.9400 g/cc. High density polyethylene (HDPE) usually has adensity in the range of 0.9400 to 0.9600 g/cc, which is prepared withZiegler-Natta type catalysts or single-site type catalysts (such asmetallocene catalysts) at low or moderate pressures. HDPE is generallypolymerized without comonomer, or alternatively with a small amount ofcomonomers with fewer short chain branches (SCB) than LLDPE. Linear lowdensity polyethylene (LLDPE) is generally prepared in the same manner asHDPE, except it incorporates a relatively higher amount of alpha-olefincomonomers. By way of example, comonomers such as 1-butene, 1-hexene, or1-octene are used to incorporate enough SCB into the otherwise linearpolymers to depress the density of resultant polymers into the range ofthat of LDPE.

Conventional Ziegler-Natta catalyzed polyethylene copolymers such asLLDPE have both a relatively broad molecular weight distribution and arelatively broad comonomer distribution in which the comonomers arepredominately incorporated into the low molecular weight polymermolecules or short polyethylene chains whereas the long polyethylenechains or high molecular weight polymer molecules do not contain ameaningful amount of comonomers. In other words, the conventionalZiegler-Natta catalyzed ethylene copolymers exhibit a heterogeneous SCBdistribution among polymer chains of different molecular weight. Thislack of compositional homogeneity is associated with severaldisadvantages including “organoleptic” problems caused by low molecularweight material and suboptimal impact strengths which are believed to becaused by the crystallinity of the homopolymer fraction.

Single-site catalysts normally produce resins with a narrow compositiondistribution in which comonomers are substantially uniformly distributedamong the polymer chains of different molecular weight. As a result,both short chain branch distribution and polymer chain distribution ofsingle-site catalyzed copolymers are known to be homogeneous.

It is well known that composition distribution affects the properties ofcopolymers. For example, extractable content, tear strength, dartimpact, heat sealing strength, and environmental stress crack resistance(ESCR) can all be affected by composition distribution. ConventionalZiegler-Natta catalyzed LLDPE exhibiting a broad compositiondistribution and broad molecular weight distribution is known to havegood processability as measured by extruder pressures and motor load. Infilm applications, conventional Ziegler-Natta catalyzed LLDPE (ZN LLDPE)exhibits good physical properties as related to tensile and tearstrengths, but shows low dart drop impact strength. Single-sitecatalyzed LLDPE (mLLDPE), having a narrow composition distribution andnarrow molecular weight distribution, is known to produce tough filmswith high dart impact and puncture properties. But the single-sitecatalyzed LLDPE exhibits adverse processability and weak film tensileproperties (e.g. MD tear strength).

As such, it is highly desirable to attain polyethylene resins thatexhibit ZN LLDPE type processability and a tear strength that is higherthan or equivalent to ZN LLDPE, and a dart impact strength which iscomparable to or better than that of mLLDPE. Theoretically, it ispossible to improve the toughness of films (e.g. MD tensile strength) byincreasing the amount of orientation in the machine direction duringfilm fabrication. However, conventional knowledge in the polyethylenefilm art suggests that by increasing the machine direction (MD)orientation in films during manufacturing, other physical properties,such as MD tear strength, will significantly decrease.

Certain advantages were known in the prior art regarding super-hexene ZNLLDPE for enhancing toughness properties such as dart impact whilemaintaining the MD tear of conventional ZN LLDPE. The molecular weightdistribution of super-hexene LLDPE is narrower than that of conventionalZN polymers but the composition distribution still resembles thatconventional ZN LLDPE. As a result, the dart impact strength is stillnoticeably lower than that of single-site catalyst-based LLDPE.

Therefore, there is a need for a new LLDPE composition that wouldexhibit a balance of good processability and desirable physicalproperties. The resins of the present invention were found to matchthese requirements, exhibiting a MD tear strength that is higher thanthat of super-hexene ZN LLDPE and a dart impact strength which iscomparable to or better than that of mLLDPE.

SUMMARY OF THE INVENTION

A Ziegler-Natta catalyzed ethylene copolymer having a novel compositiondistribution with superior physical properties, a process for making thesame, and articles made of this composition are provided. The resins ofthe present invention exhibit a distinctive molecular structureencompassing all the desirable attributes of both ZN catalyzedcopolymers and single-site catalyzed copolymers. In one embodiment ofthe present invention, the resins of the present invention exhibit adistinctive molecular structure in which comonomers are incorporatedinto the high molecular weight polymer molecules and distributed evenlyamong the entire polyethylene chains with substantial absence of lowmolecular weight polymer molecules. The resins of the present inventionexhibit a global composition distribution that is comparable to typicalsingle-site catalyzed polymers, with a distinctive melting behaviorwhich differs substantially from that of the single-site catalyst. Theresins having a novel composition distribution of this invention exhibita melting point of about 125° C. over the density range of 0.9140 to0.9250 g/cc, which is substantially higher in comparable density andnarrower for a given range than those of the single-site catalyyzedcopolymers respectively. The resins of the present invention have acontrolled molecular weight distribution which is narrower thanconventional ZN-copolymers but broader than single-site catalyzedcopolymers. The resins of the present invention are characterized byside chain structure sequence analysis with ¹³C-NMR experimentation, byshort chain branching distribution across molecule weight distributionwith high temperature GPC coupling with FTIR detector, by molecularweight and comonomer content analysis of each fraction obtained fromTemperature Rising Elution Fractionation (TREF) experiments with GPC andFTIR or ¹³C-NMR, by molecular weight distribution (Mw/Mn) in GPCexperiment, by polymer crystallinity and melting point analysis with DSCexperiment, and by film physical properties analysis in blown filmlines.

In another embodiment, a process to produce this novel copolymer mayinclude polymerizing ethylene and at least one alpha-olefin bycontacting the ethylene and at least one alpha-olefin with aZiegler-Natta type catalyst in a gas phase reactor at a reactor pressureof between 0.5 and 70 bar and a reactor temperature of between 20° C.and 150° C. to form an ethylene alpha-olefin copolymer. The resultingethylene alpha-olefin copolymer may have a density of 0.870 g/cc orhigher, a melt index ratio (I₂₁/I₂) between 10 and 50, a molecularweight distribution (Mw/Mn) of 2.5-8.0 and a ratio (Mz/Mw) of z-averagemolecular weight (Mz) to weight average molecular weight (Mw) of greaterthan 2.5, and a novel short chain branch distribution.

In yet another embodiment, polyethylene films having superior physicalproperties and methods for making the same are provided. Resins madeaccording to this invention that are fabricated into films or sheets bymeans of various conversion processes, including but not limited toblown film and cast film processes, have the processability, stiffnessand tear strength of conventional copolymers combined with the dartimpact and toughness strength of single-site catalyzed copolymers.

In yet another embodiment, the resins having a novel compositiondistribution of this invention exhibit a unique correlation betweenpolymer density and polymer melting point. This correlation was found tobe substantially more level than that of the single-site catalyzedcopolymers. Such a unique melting point profile would make the productsof this novel composition distribution withstand more reactor upsets andprovide a more stable operation in a gas phase reactor. The resins ofthe present invention exhibit a melting point range of about 124° C. toabout 126° C. for LLDPE polymer, which is substantially higher than thatof the single-site catalyzed copolymers of comparable density.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents the novel short chain branching distribution (SCBD) ofSample 1 and that of commercial super-hexene ZN LLDPE.

FIG. 2 presents the novel short chain branching distribution (SCBD) ofSample 1 and that of commercial mLLDPE.

FIG. 3 presents the novel short side chain branching distribution (SCBD)of Sample 1 and that of conventional ZN LLDPE.

FIG. 4 presents the TREF soluble fractions over weight average molecularweight (Mw) of LLDPE of this invention to that of mLLDPE.

FIG. 5 compares the melting point over density of the present inventionto those of conventional ZN LLDPE, super-hexene ZN LLDPE, and mLLDPE.

DETAILED DESCRIPTION

The present invention relates to the production of Ziegler-Nattacatalyzed ethylene alpha-olefin copolymers with a novel compositiondistribution which substantially combine the desirable attributes of ZNcatalyzed copolymers and single-site catalyzed copolymers. The resins ofthe present invention exhibit a composition distribution which iscomparable to single-site catalyzed copolymers but accompanied with adistinctive melting behavior which differs substantially from that ofthe single-site catalyzed copolymers. The resins of the presentinvention have a controlled molecular weight distribution which isnarrower than conventional ZN copolymers but broader than single-sitecatalyzed copolymers.

The present invention also relates to Ziegler-Natta catalysts andpolymerization processes for producing a Ziegler-Natta polyethylenehaving a density of about 0.870 g/cc or higher with balanced compositiondistribution as well as superior physical properties. The resultingethylene alpha-olefin copolymers exhibit a desirable balance ofprocessability and physical properties. The resins of the presentinvention exhibit a tear strength which is higher than ZN copolymers anda dart impact strength which is comparable to or better than single-sitecatalyzed copolymers.

In another embodiment, the resins of the present invention exhibit aunique correlation between polymer density and polymer melting pointwhich is substantially more level than that of the single-site catalyst.Such a unique profile would make products of this novel compositionwithstand more reactor upsets and provide a more stable operation in agas phase reactor. The resins of the present invention exhibit a meltingpoint range of about 124° C. to about 126° C. for LLDPE polymers, whichis substantially higher than that of the mLLDPE of comparable density.

In yet another embodiment, the resins of the present invention have amolecular weight distribution, a weight average molecular weight tonumber average molecular weight (M_(w)/M_(n)) of greater than 2.5 toabout 8.0, particularly greater than 2.5 to about 4.5, more preferablybetween about 3.0 to about 4.0, and most preferably between about 3.2 toabout 3.8. The polymers have a ratio (Mz/Mw) of z-average molecularweight (Mz) to weight average molecular weight of greater than 2.5. Thisratio is preferably between about 2.5 to about 3.8, more preferablybetween about 2.5 to about 3.5, and most preferably between about 2.5 toabout 3.0.

Composition Distribution and Film Properties

The composition distribution of an ethylene alpha-olefin copolymerrefers to the distribution of comonomers among the molecules (shortchain branch distribution) that comprise the polyethylene polymers.Conventional Ziegler-Natta catalysts and chromium based catalysts, owingto the nature of their multiple active sites, typically produce resinshaving both broad molecular weight distribution and broad compositiondistribution. These conventional Ziegler-Natta and chromium-based broadcomposition distribution resins are further characterized by comonomersincorporated predominantly in low molecular weight chains. Therefore,resins made with conventional Ziegler-Natta type catalysts have goodprocessability, high stiffness and tear strength, but weak filmtoughness properties (e.g. dart impact and puncture properties).Examples are described in U.S. Pat. Nos. 4,438,238; 4,612,300;6,172,173, 6,713,189; and 6,355,359.

Certain Single-site catalysts are capable of producing resins withnarrow composition distribution in which the comonomer content issubstantially uniform among the polymer chains of different molecularweight. These Single-site based narrow composition distribution resinsare further characterized by a very narrow molecular weightdistribution. Both short chain branch distribution and polymer chaindistribution are homogeneous due to the fact that comonomers areuniformly distributed among polymers of different molecular weight.These mLLDPEs, having both narrow composition distribution and narrowmolecular weight distribution, are known to produce tough films withhigh dart impact and puncture properties but “poor” processability, lowstiffness, and low tear strength. Examples are described in U.S. Pat.Nos. 4,937,299; 4,935,474; and WO 90/03414.

A novel composition distribution and a process for making the same areprovided in this invention. The resins of the present invention exhibit,in addition to good processability, a superior balance of physicalproperties such as stiffness and tear strength of ZN copolymers coupledwith the dart impact and other toughness strength of a single-sitecatalyzed copolymers. More specifically, the resins of the presentinvention exhibit a distinctive molecular structure in which comonomersare incorporated into the high molecular weight polymer molecules anddistributed evenly among the entire polyethylene chains with substantialabsence of low molecular weight polymer molecules. The of the presentinvention exhibit a composition distribution which is comparable tosingle-site catalyzed copolymers but with a distinctive melting behaviorthat differs substantially from that of the single-site catalyst. Theresins of the present invention exhibit a melting point of about 125° C.over the density range of 0.9140 to 0.9250 g/cc, which is substantiallyhigher in comparable density and narrower for a given range than thoseof mLLDPE. The resins of the present invention have a controlledmolecular weight distribution which is narrower than conventional ZNcatalyzed copolymers but broader than single-site catalyzed copolymers.

The distribution of the short chain branches can be measured, forexample, using Temperature Raising Elution Fractionation (TREF) inconnection with a Light Scattering (LS) detector in GPC to determine theweight average molecular weight of the molecules eluted from the TREF.The combination of TREF and GPC-LS and FTIR yields information about thebreadth of the composition distribution and whether the comonomercontents increases, decreases, or is uniform across the chains ofdifferent molecular weights. The resins of the present invention have“balanced” short chain branch distribution as shown in FIG. 1, FIG. 2and FIG. 3, which are comparable to mLLDPE but differ from super-hexeneZN LLDPE as well as conventional ZN LLDPE. However, as shown in FIG. 4,the resins of the present invention exhibit a TREF fractionationdistribution which is noticeably different from that of mLLDPE. Theresins of the present invention display a broader and more evenlydistributed TREF fractionation distribution than that of single-sitecatalysts. But unlike conventional LLDPE, the molecular weight (Mw) ofeach TREF soluble fraction of this invention was found comparable amongone another and with substantial absence of low molecular weight polymermolecules. As shown in FIG. 5, the resins of the present inventionexhibit a unique correlation between polymer density and polymer meltingpoint which is comparable to conventional LLDPE but substantially morelevel than that of mLLDPE.

The polyethylene films having a superior balance of physical propertiesand a method for making the same are provided herein. Resins of thepresent invention that are fabricated into films or sheets by means ofvarious conversion processes including, but not limited to, blown filmand cast film processes, have the processability, stiffness and tearstrength of a conventional ZN catalyzed copolymers combined with thedart impact and toughness strength of single-site catalyzed copolymers.

Catalyst Components and Catalyst Systems

The catalyst as described herein is an advanced Ziegler-Natta catalystwhich was modified with non-single-site catalyst ligands and/or interiordonor with a strong Lewis base such as aromatic compounds containingnitrogen atom. Examples are described in U.S. Pat. Nos. 6,992,034 and7,618,913.

In one embodiment, the following process may be performed in-situ toform a catalyst precursor. The process includes combining magnesiummetal; a compound having the formula R¹ _(m)Si(OR²)_(n), wherein R¹ andR² are C₁-C₂₀ hydrocarbyl, m=0-3, n=1-4, and m+n=4, and wherein each R¹and each R² may be the same or different; a compound having the formulaR³ _(x)SiX_(y), wherein R³ is C₁-C₂₀ hydrocarbyl, X is halogen, x=0-3,y=1-4, and x+y=4, and wherein each X and each R³ may be the same ordifferent; a compound having the formula MX₄, wherein M is a earlytransition metal such as Ti, Zr, or V; a compound having the formulaM(OR⁴)_(a)X_(4-a), wherein M is a early transition metal such as Ti, Zr,V, wherein R⁴ is C₁-C₂₀ hydrocarbyl, X is halogen, and 0≦a≦4; asubstituted aromatic compound containing nitrogen such as2,6-dimethylpyridine; and an alkyl halide or aromatic halide compoundhaving the formula R⁵X.

As an illustrative embodiment, a catalyst precursor formation process isdepicted with the following chemical equation:

The initial reaction temperature is typically from about 20 to about200° C. and the reaction time is from about 0.5 to about 20 hours. Morepreferably, the initial reaction temperature is from about 75 to about90° C. and the reaction time is from about 0.5 to about 1 hour. Then thereaction temperature normally is typically from about 20 to about 150°C. and the reaction time is typically about 0.5 to about 20 hours; morepreferably, the temperature is about 75 to about 85° C. and the reactiontime is from about 2 to about 4 hours. Another embodiment of the presentinvention may have a reaction time of about 3 hours to about 6 hours atabout 80° C.

In one embodiment of the present invention, the molar ratio of compoundR³ _(x)SiX_(y) to MX₄ is typically about 0.1 to about 10, and preferablyabout 0.2 to about 2.5. The molar ratio of R¹ _(m)Si(OR²)_(n) to Mg istypically about 0.01 to about 10, and preferably about 0.05 to about2.5. The tetra-alkoxysilane compound to Mg molar ratio is typicallyabout 0.01 to about 10, and preferably about 0.05 to about 2.5.

In one embodiment of the present invention, the ratio of transitionmetal compounds to Mg is typically about 0.01 to about 1, and preferablyabout 0.02 to about 0.5. The molar ratio of transition metal compound(such as 2,6-dimethylpyridine) is typically about 0.1 to about 5,preferably about 0.3 to about 1.5, and more preferably about 0.5 toabout 1.0.

Solvents used in the present invention include aliphatic hydrocarbonssuch as hexane, heptane, octane, or decane; aromatic hydrocarbons suchas toluene or xylene; alicyclic hydrocarbons such as cyclohexane,methylcyclohexane, or decalin; and ethers such as diethyl ether,diisopropyl ether, di-n-butyl ether, di-iso-butyl ether, diisoamylether, diallyl ether, tetrahydrofuran (THF), or anisole. Particularlypreferred solvents are dibutyl ether, diisoamyl ether, hexane, heptane,toluene, and xylene, used either alone or as mixed solvents, dependingon the specific reaction.

Any form of magnesium metal can be used in the present invention, but apreferred magnesium source is a finely divided metallic magnesium suchas magnesium powder. The magnesium is heated under nitrogen prior to useto obtain a fast reaction. A small amount of iodine, alkyl-alcohol,and/or alkylhalide can be used to initiate or facilitate the reactionbetween the magnesium and alkyl/aromatic halide. An organomagnesiumcompound may also be employed instead of metallic magnesium, which hasthe empirical formula RMgX and/or RMgR', where R and R′ are the same ordifferent C₂-C₁₂ alkyl groups, preferably C₄-C₁₀ alkyl groups, morepreferably C₄-C₈ alkyl groups, and most preferably both R and R′ arebutyl groups, and X is halogen.

Exemplary alkoxysilane compounds have a formula of R¹ _(m)Si(OR²)_(n),Examples of preferred alkoxysilane compounds include:tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane,tetraisobutoxysilane, tetraphenoxysilane, tetra(p-methylphenoxy)silane,tetrabenzyloxysilane, methyl-trimethoxysilane, methyltriethoxysilane,methyltributoxysilane, methyltriphenoxysilane, methyltriphenoxysilane,ethyltriethoxysilane, ethyltriisobutoxysilane, ethyl-triphenoxysilane,butyltrimethoxysilane, butyltriethoxysilane, butyltributoxysilane,butyltriphenoxysilane, isobutyltriisobutoxysilane, vinyltriethyoxysilane, allyl-trimethoxysilane, phenyltrimethoxysilane,phenyltriethoxysilane, benzyl-triphenoxysilane, methyltriallyloxysilane,dimethyldimethoxysilane, dimethyl-diethoxysilane,dimethyl-diisopropyloxysilane, dimethyldibutoxysilane,dimethyldihexyloxysilane, dimethyldiphenoxy-silane,diethyldiethoxysilane, diethyl-diisobutoxysilane,diethyldiphenoxysilane, dibutyl-diisopropyloxysilane,dibutyl-dibutoxysilane, dibutyldiphenoxysilane,diisobutyldiethoxysilane, diisobutyl-diisobutoxysilane,diphenyldimethoxysilane, diphenyldiethoxysilane,diphenyl-dibutoxysilane, dibenzyldiethoxysilane, divinyldiphenoxysilane, diallyldipropoxysilane, diphenyldiallyloxysilane,methylphenyldimethoxysilane, chlorophenyldiethyoxysilane,polymethylhydrosiloxane, polyphenylhydrosiloxane, and combinationsthereof.

Halogen-substituted silane has the formula R³ _(x)SiX_(y), wherein R¹ isC₁-C₂₀ hydrocarbyl or substituted hydrocarbyl, X is halogen, x is 0-3, yis 1-4, and x+y=4. More than one hydrocarbyl group may be employed inthe halogen-substituted silane, and more than one halogen may beemployed in the halogen-substituted silane. Preferredhalogen-substituted silane compounds include: silicon tetrachloride,tetrabromosilane, tetrafluorosilane, tetrachlorosilane,allyldichlorosilane, allyltrichlorosilane, benzyltrichlorosilane,bis(dichlorosilyl)methane, 2-bromoethyltrichlorosilane,t-butyldichloro-silane, t-butyltrichlorosilane,2-(carbomethoxy)ethyltrichlorosilane,2-chloroethylmethyl-dichlorosilane, 2-chloroethyltrichlorosilane,1-chloroethyltrichlorosilane, chloromethylmethyl-dichlorosilane,((chloromethyl)phenylethyl)trichlorosilane, chloromethyltrichlorosilane,2-cyanoethylmethyldichlorosilane, cyclohexyl-trichlorosilane,cyclopentyltrichlorosilane, cyclotetramethylenedichlorosilane,cyclo-trimethylenedichlorosilane, decylmethyldichlorosilane,dibenzyloxydichlorosilane, 1,5-dichlorohexamethyltrisiloxane,(dichloromethyl)trichlorosilane, dichlorosilane,1,3-dichloro-tetramethyldisiloxane, diethoxydichlorosilane,ethylmethyl-dichlorosilane, ethyltrichlorosilane, heptyltrichlorosilane,hexachlorodisilane, hexachloro-disiloxane, isobutyltrichlorosilane,methyltrichlorosilane, octyltrichlorosilane, pentyltrichlorosilane,propyltrichlorosilane, and trichloromethyltrichlorosilane.

The halogenized transition metal compounds have chemical formulas of MX₄and M(OR⁴)₄, wherein R⁴ is typically a hydrocarbon group having 1 to 20carbon atoms, X is a halogen atom. The compound may contain a transitionmetal M from the Group 4 or 5 transitional metals as identified by theperiodic table of the elements. M is preferably selected from Ti, Zr,and Hf. Mixtures of Group 4 and 5 transition metal compounds, preferablyof titanium and vanadium, may be employed to control molecular weightand molecular weight distribution of the polymers produced. Of thesecompounds, Ti compounds (e.g., TiX₄ and Ti(OR)₄) are preferred. Thetransition metal compound employed in the present invention preferablyis a halide, hydrocarbyloxide, or mixed halide/hydrocarbyloxide oftitanium, zirconium, hafnium, vanadium.

Preferred R groups in the formula M(OR⁴)₄ include alkyl groups such asmethyl, ethyl, propyl, i-propyl, butyl, i-butyl, amyl, i-amyl, hexyl,heptyl, octyl, decyl and dodecyl groups; aryl groups such as phenyl,cresyl, xylyl and naphthyl groups; cycloalkyl groups such as cyclohexyland cyclopentyl groups; alkenyl groups such as an allyl group; andaralkyl groups such as a benzyl group. Among these, alkyl groups having2 to 18 carbon atoms and aryl groups having 6 to 18 carbon atoms areparticularly suitable, and straight-chain alkyl groups having 2 to 18carbon atoms are particularly suitable.

Preferred Ti(OR⁴)₄ compounds include: tetra-n-butoxytitanium,tetra-isobutoxytitanium, tetra-sec-butoxytitanium,tetra-tert-butoxytitanium, tetra-n-pentyloxytitanium,tetracyclopentyloxytitanium, tetra-n-hexyloxytitanium,tetracyclohexyloxytitanium, tetra-n-heptyloxytitanium,tetra-n-octyloxy-titanium, tetra-2-ethylhexyloxytitanium,tetranonyloxytitanium, tetradecyloxytitanium, tetraisobornyloxy-titanium, tetra-oleyloxytitanium, tetraallyloxytitanium,tetrabenzyloxytitanium, tetrabenzhydryloxytitanium,tetraphenoxytitanium, tetra-o-methylphenoxytitanium,tetra-m-methylphenoxytitanium, tetra-1-naphthyloxytitanium,tetra-2-naphthyloxytitanium, and mixtures thereof.

Examples of preferred aromatic compounds containing nitrogen areelectron donors with strong Lewis base, selected from nitrogen-basedcompounds such as 2, 6-dimethylpyridine.

The substituted aromatic ring nitrogen compound is preferably employedin amounts sufficient to have a molar ratio of substituted aromatic ringnitrogen compound to transition metal compound (as added in the previousprocessing step) of typically from about 0.010:1 to about 50:1,preferably from about 0.02:1 to about 10:1, and more preferably fromabout 0.1:1 to about 5:1. Although the conditions are not generallycritical, one acceptable procedure is to heat at about 80° C. for about30 minutes to about 100 minutes, preferably about 60 minutes, until thedesired temperature is obtained to yield third reaction complex C, whichis generally a yellow/dark brown. The third reaction complex C ispreferably used, for the following steps in situ without furtherseparation or characterization.

Preferred substituted aromatic ring nitrogen compounds includesubstituted dipyridyl, pyrimidine, pyrazine, and terpyridine compounds,such as: 2,2′-dipyridyl, 6,6% dimethyl-2,2′-dipyridyl, 2,2′-diquinolyl,4-(p-tolyl)-2,2′:6′,2″-terpyridine, 2,6-dimethylpyridine,2,6-diisopropylpyridine, 2,6-ditertbutylpyridine,2,4,6-trimethylsilylpyridine, 2,6-dimethoxypyridine,2,6-bis(chloromethyl)-pyridine, 2,6-dimethypyrazine,2,3,5-trimethylpyrazine, 2,4,6-trimethyl-s-triazine,2,3,5,6-tetramethylpyrazine, pyrimidine, pyrazine, pentafluoropyridine,pentachloropyridine, 2,4,6-trimethylpyrimidine, 3-methylpyridazine,2,6-dimethylpyridazine, 2,6-pyridinecarboxylic acid,2,6-pyridinediacetate, 2,6-pyridinecarbonyl dichloride,2,6-pyridinecarboxaldehyde, 2,6-pyridinedicarboxamide,2,6-pyridinedimetanol, 2,6-pyridinediethanol, 2,6-diacetylpyridine,2,6-Bis(chloromethyl)pyridine, 2,6-Bis(bromomethyl)pyridine,2,6-pyridinecarbonitrile, and mixture thereof.

The alkyl or aromatic halide preferably has the formula R⁵X, wherein R⁵is an alkyl group containing 3 to 20 carbon atoms or an aromatic groupcontaining 6 to 18 carbon atoms, and X is preferably chlorine orbromine. Preferred alkyl and aromatic halides include: n-propylchloride, propyl bromide, iso-propyl chloride, iso-propyl bromide,n-butyl chloride, n-butyl bromide, sec-butyl chloride, sec-butylbromide, tert-butyl chloride, tert-butyl bromide, iso-amyl chloride,iso-amyl bromide, n-hexyl chloride, n-hexyl bromide, n-octyl chloride,n-octyl chloride, 2-ethylhexyl chloride, 2-ethylhexyl chloride,chlorobenzene, bromobenzene, and iodinebenzene. The Mg/RX molar ratio istypically about 0.2 to about 2, and preferably about 0.5 to about 1.2.

Catalyst System

The catalyst component of the present invention can be combined with anorgano-aluminum compound to form a solid catalyst system for thepolymerization of alpha-olefins. As explained herein, the solid catalystsystem can be used for solution, slurry, and gas phase polymerizationprocesses. Depending on the process, the catalyst can be introduced as asolid, with or without an inert support, or injected in the reactionzone in a pre-polymer form. Supported catalyst and pre-polymers aremostly indicated for gas phase and slurry processes.

The catalyst composition prepared as described above is filtered andwashed, preferably once or more at a temperature of about 50° to 120°C., with a hydrocarbon (e.g., hexane), and then dried at 25° to 75° C.for about 1-5 hours.

The catalyst composition may be activated in situ by adding theco-catalyst and the solid catalyst composition separately to thepolymerization medium. It is also possible to combine the catalystcomposition and the co-catalyst before introduction into thepolymerization medium, e.g., for up to about 2 hours at a temperaturefrom about −40° to about 100° C. A suitable activating amount of theco-catalyst may be used. The number of moles of the co-catalyst per gramatom of titanium in the catalyst may be from about 0.05 to about 500.

Preferred co-catalysts include: organometallic compounds, for example,trialkylaluminum compounds such as trimethylaluminum, triethylaluminum,tri(n-propyl)aluminum, tri(isopropyl)aluminum, tri(n-butyl)aluminum,tri(isobutyl)aluminum, tri(t-butyl)aluminum, trihexylaluminum,triamyl-aluminum, and tri(n-octyl)aluminum; dialkylaluminum hydridessuch as diisobutylaluminum hydride; dialkylaluminum halides such asdimethylaluminum chloride, diethylaluminum chloride, diisobutylaluminumchloride, di(t-butyl)aluminum chloride and diamylaluminum chloride;alkylaluminum dihalides such as methylaluminum dichlorides,ethylaluminum dichloride, isobutylaluminum dichloride, t-butylaluminumdichloride and amylaluminum dichloride; dialkylaluminum alkoxides suchas diethylaluminum ethoxide; and alkylalumoxanes such astetraethyldialumoxane, tetrabutyldialumoxane, methylalumoxane andethylalumoxane. Among these organometallic compounds, trialkylaluminum,the mixture of the trialkylaluminum and dialkylaluminum halide, andalkylalumoxane are preferred, with trimethylaluminum, triethylaluminum,tri-iso-propylaluminum, and tri(n-octyl)aluminum being more preferredactivators.

Polymerization/Copolymerization Process and Polymer Products

Ethylene and alpha-olefins may be copolymerized with the catalystsystems prepared accordance with the teachings of the present inventionby any suitable process. Suitable polymerization processes includeslurry phase, solution, gas phase, and a high pressure process, or anycombination thereof. A desirable process is a gas phase polymerizationof one or more one or more olefin monomers having from 2 to 30 carbonatoms, preferably from 2 to 12 carbon atoms, and more preferably from 2to 8 carbon atoms. The ethylene copolymers prepared in accordance withthe teachings of the present invention may be copolymers of ethylenewith one or more C₃-C₁₀ alpha-olefins. Thus, copolymers having two typesof monomeric units are possible as well as terpolymers having threetypes of monomeric units. Particular examples of such polymers includeethylene/1-butene copolymers, ethylene/1-hexene copolymers,ethylene/1-octene copolymers, ethylene/4-methyl-1-pentene copolymers,ethylene/1-butene/1-hexene terpolymers, ethylene/propylene/1-hexeneterpolymers and ethylene/propylene/1-butene terpolymers. More preferredco-monomers are 4-methyl-1-pentene, 1-hexene, 1-octene and 1-butene forthe catalyst prepared according to the present invention.

There are no particular restrictions on the polymerization conditionsfor production of polyolefins by the method of the invention, such asthe polymerization temperature, polymerization time, polymerizationpressure, monomer concentration, etc., but typically the polymerizationtemperature is from about −100° to about 300° C., the polymerizationtime is from about 10 seconds to about 20 hours, and the polymerizationpressure is typically from normal pressure to about 350 psi. Hydrogen orthe like may be used to adjust the molecular weight duringpolymerization. The polymerization may be carried out in a batch system,semi-continuous system, or continuous system, and it may be carried outin one or more stages under different polymerization conditions. Thepolyolefins may be directly obtained from a gas phase process, orobtained by isolation and recovery of solvent from a slurry or solutionprocess.

In one embodiment of the present invention, the solid catalystcomposition may be subjected to pre-polymerization, thereby obtaining apre-polymerized catalyst component, which is then used for gas phasepolymerization. In carrying out the pre-polymerization, for example, thesolid catalyst component and an organoaluminum compound are contactedwith an olefin. Examples of the olefin used for the pre-polymerizationare ethylene, propylene and butene-1. The pre-polymerization may beeither homopolymerization or copolymerization. In thepre-polymerization, it may be preferable to make a slurry containing thesolid catalyst component using a solvent. Examples of suitable solventsinclude aliphatic hydrocarbons such as butane, pentane, hexane andheptane, and aromatic hydrocarbons such as toluene and xylene. Theslurry concentration is typically from about 0.001 to 0.3 g-solidcatalyst component/10 ml solvents, and preferably from about 0.02 toabout 0.2 g-solid catalyst component/10 ml-solvent. The organoaluminumcompound may be used in an amount typically from about 0.1 to about 100,and preferably from about 0.5 to about 50, calculated as the Al/Tiatomic ratio, i.e., the atomic ratio of the Al atom in theorganoaluminum compound to the Ti atom in the solid catalyst component.The temperature for the pre-polymerization is typically from about −30°to about 100° C., and preferably from about −10° to 85° C. Yield of thepre-polymer is typically about 0.1 to 500 g, and preferably about 0.5 toabout 50 g per mmol of Ti. When used for gas phase polymerization, thepre-polymerized solid catalyst component obtained may be combined withan inert diluent to form slurry, or dried to obtain a flowing powder.The prepolymer is injected into a gas phase reactor for furtherpolymerization to produce a given LLDPE product.

Typically in a gas phase polymerization process a continuous cycle isemployed wherein one part of the cycle of a reactor system, a cyclinggas stream, otherwise known as a recycle stream or fluidizing medium, isheated in the reactor by the heat of polymerization. This heat isremoved from the recycle composition in another part of the cycle by acooling system external to the reactor. Generally, in a gas fluidizedbed process for producing polymers, a gaseous stream containing one ormore monomers in continuously cycled through a fluidized bed in thepresence of a catalyst or prepolymer under reactive conditions. Thegaseous stream is withdrawn from the fluidized bed and recycled backinto the reactor. Simultaneously, polymer product is withdrawn from thereactor and fresh monomer is added to replace the polymerized monomer.

The ethylene partial pressure should vary between 10 and 250 psi,preferably between 65 and 150 psi, more preferably between 75 and 140psi, and most preferably between 90 and 120 psi. More importantly, aratio of comonomer to ethylene in the gas phase should vary from 0.0 to0.50, preferably between 0.005 and 0.25, more preferably between 0.05and 0.10, and most preferably between 0.10 and 0.15. Reactor pressuretypically varies from 100 psi to 500 psi. In one aspect, the reactorpressure is maintained within the range of from 200 psi to 500 psi. Inanother aspect, the reactor pressure is maintained within the range offrom 250 psi to 350 psi.

The catalysts prepared according to the present invention areparticularly useful for the production of copolymers. Such copolymerresins may have a density of 0.958 g/cc or less, preferably 0.952 g/ccor less, or more preferably 0.940 g/cc or less. In accordance withcertain aspects of the present invention, it is possible to achievedensities of less than 0.910 g/cc and even as low as 0.870 g/cc.Copolymer resins produced in accordance with the present inventionpreferably contain at least about 75 percent by weight of ethyleneunits. Preferably, the copolymer resins of the present invention containat least 0.5 weight percent, for example from 0.5 to 25 weight percentof an alpha-olefin.

The molecular weight of the copolymers may be controlled in a knownmanner, preferably by using hydrogen. With the catalysts producedaccording to the present invention, molecular weight may be suitablycontrolled with hydrogen when the polymerization is carried out attemperatures from about 20° C. to about 300° C. This control ofmolecular weight may be evidenced by a measurable positive change of themelting index (I₂).

The molecular weight distribution (MWD) of the polymers preparedaccording to the present invention, as expressed by the MFR values,varies from about 10 to about 40. MFR is the ratio of the high-load meltindex (HLMI or I₂₁) to the melt index (MI or I₂) for a given resin(MFR=I₂₁/I₂). The ethylene/1-hexene copolymer having a density of 0.910g/cc to 0.930 g/cc, in a preferred embodiment, has a melt index ratio(I₂₁/I₂) of between about 20 to about 30.

The polymers of the present invention have a molecular weightdistribution, a weight average molecular weight to number averagemolecular weight (M_(w)/M_(n)), of between about 2.5 to about 8.0,preferably between about 2.5 to about 4.5, more preferably between about3.0 to about 4.0, and most preferably between about 3.2 to about 3.8.The polymers have a ratio (Mz/Mw) of z-average molecular weight (Mz) toweight average molecular weight of greater than 2.5. In one embodiment,this ratio is from about 2.5 and 3.8. In yet another embodiment, thisratio is from about 2.5 to about 3.5. In still yet another embodiment,this ratio is from about 2.5 to about 3.0. The ratio of z-averagemolecular weight to weight average molecular weight (Mz/Mw) reflects theinter- and/or intro-macromolecular entanglement and unique polymerrheology behavior.

Copolymer Compounding/Extrusion and LLDPE Pellets

The copolymers produced according to the teachings of the presentinvention may also be blended with additives to form compositions thatcan then be used in articles of manufacture. Those additives includeantioxidants, nucleating agents, acid scavengers, plasticizers,stabilizers, anticorrosion agents, blowing agents, other ultravioletlight absorbers such as chain-breaking antioxidants, etc., quenchers,antistatic agents, slip agents, pigments, dyes and fillers and cureagents such as peroxide. These and other common additives in thepolyolefin industry may be present in polyolefin compositions from 0.01to 50 wt % in one embodiment, and from 0.1 to 20 wt % in anotherembodiment, and from 1 to 5 wt % in yet another embodiment, wherein adesirable range may comprise any combination of any upper wt % limitwith any lower wt % limit.

In particular, antioxidants and stabilizers such as organic phosphitesand phenolic antioxidants may be present in the polyolefin compositionsfrom 0.001 to 5 wt % in one embodiment, and from 0.01 to 0.8 wt % inanother embodiment, and from 0.02 to 0.5 wt % in yet another embodiment.Non-limiting examples of organic phosphites that are suitable aretris(2,4-di-tert-butylphenyl)phosphite (IRGAFOS 168) and tris(nonylphenyl) phosphite (WESTON 399) Non-limiting examples of phenolicantioxidants include octadecyl 3,5 di-t-butyl-4-hydroxyhydrocinnamate(IRGANOX 1076) and pentaerythrityltetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (IRGANOX 1010);and 1,3,5-Tri(3,5-di-tert-butyl-4-hydroxybenzyl-isocyanurate (IRGANOX3114).

Fillers and fatty acid salts may also be present in the polyolefinincluding LLDPE compositions. Filler may be present from 0.1 to 65 wt %in one embodiment, and from 0.1 to 45 wt % of the composition in anotherembodiment, and from 0.2 to 25 wt % in yet another embodiment. Desirablefillers include but not limited to titanium dioxide, silicon carbide,silica (and other oxides of silica, precipitated or not), antimonyoxide, lead carbonate, zinc white, lithopone, zircon, corundum, spinel,apatite, Barytes powder, barium sulfate, magnesiter, carbon black,dolomite, calcium carbonate, talc and hydrotalcite compounds of the ionsMg, Ca, or Zn with Al, Cr or Fe and CO₃ and/or HPO₄, hydrated or not;quartz powder, hydrochloric magnesium carbonate, glass fibers, clays,alumina, and other metal oxides and carbonates, metal hydroxides,chrome, phosphorous and brominated flame retardants, antimony trioxide,silica, silicone, and blends thereof. These fillers may particularlyinclude any other fillers and porous fillers and supports known in theart.

Fatty acid salts may be present from 0.001 to 6 wt % of the compositionin one embodiment, and from 0.01 to 2 wt % in another embodiment.Examples of fatty acid metal salts include lauric acid, stearic acid,succinic acid, stearyl lactic acid, lactic acid, phthalic acid, benzoicacid, hydroxystearic acid, ricinoleic acid, naphthenic acid, oleic acid,palmitic acid, and erucic acid, suitable metals including Li, Na, Mg,Ca, Sr, Ba, Zn, Cd, Al, Sn, Pb and so forth. Desirable fatty acid saltsare selected from magnesium stearate, calcium stearate, sodium stearate,zinc stearate, calcium oleate, zinc oleate, and magnesium oleate.

In the physical process of producing the blend of polyolefin and one ormore additives, sufficient mixing should take place to assure that auniform blend will be produced prior to conversion into a finishedproduct. The polyolefin can be in any physical form when used to blendwith the one or more additives. In one embodiment, reactor granules,defined as the granules of polymer that are isolated from thepolymerization reactor, are used to blend with the additives. Thereactor granules have an average diameter of from 10 μM to 5 mm and from50 μM to 10 mm in another embodiment. Alternately, the polyolefin is inthe form of pellets, such as, for example, having an average diameter offrom 1 mm to 6 mm that are formed from melt extrusion of the reactorgranules.

One method of blending the additives with the polyolefin is to contactthe components in a tumbler or other physical blending means, thepolyolefin being in the form of reactor granules. This can then befollowed, if desired, by melt blending in an extruder. Another method ofblending the components is to melt blend the polyolefin pellets with theadditives directly in an extruder, Brabender or any other melt blendingmeans.

Film Extrusion and Film Properties

The polymers produced are more easily extruded into film products bycast or blown film processing techniques as compared to commercialSuper-hexene (I) and (II), commercial octene-1 LLDPE (I) and (II), andcommercial mLLDPE (I), (II) and (III) of comparable melt index anddensity. The resins in this invention have, for a comparable MI, a MWDnarrower than hexene copolymer resins but broader than mLLDPEs. Theresins made from this invention also exhibit a molecular structure suchas comonomer distribution very similar to typical mLLDPE resins.

More specifically, in the present invention, using Davis-Standard blownfilm pilot line evaluates the film extrusion processability and filmperformance of new LLDPE with novel composition distribution against theindustry leading products: i.e., SC₆, C₈ and mLLDPE products. The filmprocess conditions in Davis-Standard blown film pilot include 90-mil diegap, at 2.5 BURs, 1-mil gages and 12 lbs/hr/in (450 lbs/hr). Film dartimpact (g/mil) was tested by ASTM D-1709, and film Elmendorf Tear(g/mil) by ASTM-D-1922 and Secant Modulus by ASTM D-882.

EXAMPLES

In order to provide a better understanding of the foregoing discussion,the following non-limiting examples are offered. Although the examplesmay be directed to specific embodiments, they are not to be viewed aslimiting the invention in any specific respect.

All parts, proportions, and percentages are by weight unless otherwiseindicated. All examples were carried out in dry, oxygen-freeenvironments and solvents. Ti, Si, and Mg content in the solid catalystcomponent were determined by ICP emission analysis method. Melt flowindex (MI) of polymer was measured at 190° C., according to ASTM D1238.Melt flow ratio (MFR), which is the ratio of high melt flow index (HLMIor I₂₁) to melt index (MI or I₂), was used as a measure of melt fluidityand a measure of the molecular weight distribution of polymer. The meltflow ratio is believed to be an indication of the molecular weightdistribution of the polymer, the higher the value, the broader themolecular weight distribution. Density was measured according to ASTM D1505-98. All molecular weights are weight average molecular weightunless otherwise noted. Molecular weights (weight average molecularweight (M_(w)) and number average molecular weight (M_(e)) and (M_(Z))were measured by Gel Permeation Chromatography (GPC). The melting pointof polymers was measured by DSC. Composition distribution or short chainbranching distribution of polymers, and comonomer content and molecularweight in each fractionated fraction were determined by TREF andGPC-FTIR at a high temperature of 145° C., flow rate of 0.9 mL/min,solvent of TCB, and the concentration of solution of 2.5 mg/mL.

Catalyst Preparation

The solid catalyst composition and properties of polymers in theexamples were measured according to the following methods:

Anhydrous hexane (2 L), magnesium (31.9 g), iodine (3.3 g), isopropanol(3.66 ml), and butyl chloride (5.8 ml) were successively charged into a5 L reactor equipped with an anchor stirrer driven by a magnetic motor.The reactor was heated to 85° C. within 60 minutes and then cooled to80° C. within 20 minutes. Tetraethoxy orthosilicate (20 ml, 89.7 mmol)and silicon tetrachloride (40 ml, 349.1 mmol) were added to the reactorand held for reaction at 80° C. for 20 minutes. Ti(OPr)₄ (46.1 ml, 166.9mmol) and TiCl₄ (18.3 ml, 166.9 mmol) were charged to the reactor undernitrogen at 80° C., and the slurry mixture was stirred for 0.5 hour,followed by the introduction of 2,6-dimethylpyridine (19.5 ml, 166.9mmol) in the hexane solution. The reaction was stirred at 80° C. for 1hour to yield a brown/yellow reaction product, which was used withoutfurther separation.

The brown/yellow reaction product was then directly supported withmagnesium/silicon composite support, which was prepared in-situ by theslow introduction of n-butyl chloride (213.3 ml, 2041.5 mmol) into thebrown/yellow reaction product over 4 hours at 80° C. After the additionof n-butyl chloride, the reaction mixture was continually stirred at 80°C. for 2 more hours and then cooled to temperature of 50° C. Theresulting precipitate was rapidly washed 3 times with 2L hexane at 50°C. A solid magnesium-based supported titanium catalyst component wasobtained. Analysis shows that the red/brown catalyst component contains7.5 wt % Ti, 2.1 wt % Si, and 14.5 wt % Mg, respectively.

Polymerization

The ethylene/1-hexene copolymers from Examples 1-6 were produced inaccordance with the following general procedure. Polymerization wasconducted in a commercial BP process gas-phase fluidized bed reactoroperating at approximately 300 psig total pressure. Fluidizing gas waspassed through the bed at a velocity of approximately 1.8 feet persecond. The fluidizing gas exiting the bed entered a resin disengagingzone located at the upper portion of the reactor. The fluidizing gasthen entered a recycle loop and passed through a cycle gas compressorand water-cooled heat exchanger. The shell side water temperature wasadjusted to maintain the reaction temperature to the specified value inthe range of from 175° F. to 195° F. Ethylene, hydrogen, 1-hexene andnitrogen were fed to the cycle gas loop just upstream of the compressorat quantities sufficient to maintain the desired gas composition. Gascompositions were measured by an on-line GC analyzer. The catalyst inthe form of prepolymer was injected to the reactor bed through astainless steel injection tube at a rate sufficient to maintain thedesired polymer production rate. Nitrogen gas was used to disperse thecatalyst into the reactor. Product was withdrawn from the reactor,polymer was collected after discharging and degassing in the downstream,gases were recycled in the loops and residual catalyst and cocatalyst inthe resin was deactivated with a wet nitrogen purge. Final powderproduct (polymer) was transferred into extrusion and pelletized intogranular product. Table 1 summarizes the reaction conditions.

Granular product for Examples 1-6 was screened and dry-blended withsuitable additives such as Irganox-1076 (available from Ciba-Geigy)1076, IR-168, TNPP, Polybloc Talc, Zinc stearate, Erucamide, and DHT-4V.Pelletizing of Examples 1-4 was carried out on a twin-screw extruderequipped with an underwater pelletizer. Output rate was approximately35,000-50,000 lb/hr and melting temperature was 231° C. (447° F.).

TABLE 1 Reaction Conditions for Examples 1-6 Examples 1 2 3 4 5 6 C2Production rate, lb/hr 38,000 40,000 45,000 48,000 50,000 50,000Operating pressure, psi 285 285 285 285 285 305 C2 partial pressure, psi90 95 100 105 105 125 C2/H2 ratio 0.245 0.241 0.235 0.230 0.250 0.230C6/C2 ratio 0.1215 0.1215 0.1215 0.1255 0.115 C4/C2 ratio 0.32 Reactiontemp., ° F. 185 185 185 185 185 185 Res. Time, hr 4.5 4.5 4.5 4.5 4.34.0 I₂, dg/min (powder) 0.80 0.8 0.8 0.8 1.0 1.0 Powder density, g/cc0.9160 0.9155 0.9158 0.9150 0.9190 0.9175Comonomer Composition Distribution

The distribution of the short chain branches can be clearly measured,for example, using Temperature Raising Elution Fractionation (TREF) andGPC to determine the weight average molecular weight of the moleculeseluted from the TREF column at a given temperature. The use of TREF,GPC-LS and FTIR yields information about the breadth of the compositiondistribution and whether the comonomer content increases, decreases, oris uniform across the chains of different molecular weights. Short chainbranching distribution (SCBD) for comparatives samples are showed inFIGS. 1, 2 and 3. The copolymers have a novel composition distributionin which commoners are incorporated into the high molecular weightpolymer molecules and distributed about evenly among the entirepolyethylene chains with substantial absence of low molecular weightpolymer molecules. The resins having a novel composition distributionexhibit a global composition distribution which is comparable tohomogeneous mLLDPE, but differ from broad composition distribution oftypically conventional ZN-LLDPE as well as Super-hexene Z-N LLDPE.However, as shown in FIG. 4, the resins having a novel compositiondistribution of this invention exhibit a TREF fractionation distributionwhich is noticeably different from that of mLLDPE. But unlike theconventional LLDPE, the molecular weight (Mw) of each TREF solublefraction of this invention was found comparable among one another andwith substantial absence of low molecular weight polymer molecules. Asshown in FIG. 5, the resins having a novel composition distribution ofthis invention exhibit a unique correlation between polymer density andpolymer melting point which is comparable to conventional LLDPE butsubstantially more level than that of the mLLDPE.

Polymer Properties and Blown Film Properties

Polymer that characterized by DSC and GPC were extruded usingDavis-Standard Blown Film Pilot Line. The film process conditions inDavis-Standard blown film pilot include 90-mil die gap, at 2.5 BURs,1-mil gages and 12 lbs/hr/in (450 lbs/hr). Extrusion melt temperaturesin extruder A, extruder B and extruder C are about 438° F., 439° F., and435° F.

Comparative Example 6 is commercial super-hexene (I). ComparativeExample 7 is commercial super-hexene (II). Comparative Example 8 iscommercial octane-1 LLDPE (I). Comparative Example 9 is commercialoctane-1 LLDPE (II). Comparative Example 10 is commercial mLLDPE (I).Comparative Example 11 is commercial mLLDPE (II). Comparative Example 12is commercial mLLDPE (III). The polymer and blown film properties andextrusion data are shown in Tables 2-4.

As shown in the Tables 2-4 above, the resins having a novel compositiondistribution exhibit a global intermolecular compositional distributionsimilar to mLLDPE, but accompanied with high distinctive crystallinityand high melting point. The resins having a novel compositionaldistribution have controlled molecular weight distribution which isnarrower than conventional ZN-LLDPE but broader than mLLDPE. The resinshaving a novel compositional distribution exhibit, in addition to a goodprocessability, a superior balance of physical properties such asstiffness and tear strength of ZN-catalyzed ethylene-alpha olefincopolymers with the toughness (e.g. dart impact) strength of single-sitecatalyzed ethyelene compolymers. More specifically speaking, the filmproperties of the resins having a novel composition distribution exhibitexcellent mechanical properties such as dart impact, MD tear, andtensile strength, superior than those of ExxonMobil and Nova superhexene products. The films have dart impact equivalent to C8 productsand single-site catalyzed PE resins in toughness such as dart impact,but having high MD tear tensile properties imported from Ziegler-Nattatype polymers. A film provided includes a 2% secant modulus of from18,000 to 28,000 psi, a heat seal strength of greater than 1300 g/inch,a dart impact resistance of greater than 500 g/mil, and MD tear strengthof at least 450 g/mil.

TABLE 2 Blown Film Properties Comparison for Ziegler-Natta C6-LLDPEComparative Comparative Example 6 Example 7 Example 1 Example 4Super-Hexene (I) Super Hexene (II) MI (I₂) dg/min 0.76 0.91 0.95 0.72MFR (I₂₁/I₂) 25 27 25 28 M_(w)/M_(n) 3.5 3.8 3.8 4.3 Resin Density(g/cc) 0.9205 0.9201 0.9215 0.9206 Melt Point (° C.) 125 125 124 123Film Gauge Target (mils) 1.00 1.00 1.00 1.00 Blow UP Ratio (BUR) 2.5 to1 2.5 to 1 2.5 to 1 2.5 to 1 TEAR STRENG MD, g/mil 465 625 413 357 TEARSTRENG TD, g/mil 647 705 712 659 DART IMPACT, g/mil 538 505 293 409 FILMHAZE, % 38 21 37 39 Tensile Str. @ Brk (MD), psi 5677 5680 5395 6379Tensile Str. @ Brk (TD), psi 4989 4293 4347 4133 Film Elongation @ Brk(MD) % 672 703 691 651 Film Elongation @ Brk (TD) % 884 870 915 844 (MD)SEC. MOD @ % STRN, % 18311 23999 18693 17500 (TD) SEC. MOD @ % STRN, %18994 29814 19837 18266 % STRAIN, % 2 1 2 1 (MD) TENSILE STR @YlD, psi1529 1656 1388 1436 (TD) TENSILE STR @YlD, psi 1533 11637 1339 1392Extrusion Parameters: Melt Temperature (° F.) 423 413 420 433 HeadPressure (psi) 3989 3724 3741 4123 Die Pressure (psi) 2425 2191 22652491 Motor Load (amps) 64.1 60.3 63.2 70.3

TABLE 3 Blown Film Properties Comparison with C8-LLDPE ComparativeComparative Example 8 Example 9 Example 1 Example 4 C8 LLDPE (I) C8LLDPE (II) MI (I₂) dg/min 0.76 0.91 0.98 0.98 MFR (I₂₁/I₂) 25 27 28 29M_(w)/M_(n) 3.5 3.8 3.7 4.1 Resin Density (g/cc) 0.9205 0.9201 0.92170.9215 Melt Point (° C.) 125 125 118 121 Film Gauge Target (mils) 1.001.00 1.00 1.00 Blow UP Ratio (BUR) 2.5 to 1 2.5 to 1 2.5 to 1 2.5 to 1TEAR STRENG MD, g/mil 465 625 355 319 TEAR STRENG TD, g/mil 647 705 709750 DART IMPACT, g/mil 538 505 200 257 FILM HAZE, % 38 21 25 26 TensileStr. @ Brk (MD), psi 5677 5680 4268 5688 Tensile Str. @ Brk (TD), psi4989 4293 2859 4470 Film Elongation @ Brk (MD) % 672 703 583 618 FilmElongation @ Brk (TD) % 884 870 697 865 (MD) SEC. MOD @ % STRN, % 1831123999 22604 21504 (TD) SEC. MOD @ % STRN, % 18994 29814 27532 22439 %STRAIN, % 2 1 2 2 (MD) TENSILE STR @YlD, psi 1529 1656 1614 1708 (TD)TENSILE STR @YlD, psi 1533 11637 1588 1482 Extrusion Parameters: MeltTemperature (° F.) 423 413 433 420 Head Pressure (psi) 3989 3724 41233741 Die Pressure (psi) 2425 2191 2491 2265 Motor Load (amps) 64.1 60.370.3 63.2

TABLE 4 Blown Film Properties Comparison with mLLDPE ComparativeComparative Comparative Example 10 Example 11 Example 12 Example 1mLLDPE (I) mLLDPE (II) mLLDPE (III) MI (I₂) dg/min 0.76 0.93 1.11 1.01MFR (I₂₁/I₂) 25 15 13 17 M_(w)/M_(n) 3.5 2.7 2.3 2.6 Resin Density(g/cc) 0.9205 0.9223 0.9174 0.9206 Melt Point (° C.) 125 118 116 118Film Gauge Target (mils) 1.00 1.00 1.00 1.00 Blow UP Ratio (BUR) 2.5 to1 2.5 to 1 2.5 to 1 2.5 to 1 TEAR STRENG MD, g/mil 465 298 205 241 TEARSTRENG TD, g/mil 647 406 380 381 DART IMPACT, g/mil 538 513 448 633Tensile Str. @ Brk (MD), psi 5677 3587 5611 3342 Tensile Str. @ Brk(TD), psi 4989 3122 5738 2439 Film Elongation @ Brk (MD) % 672 528 603532 Film Elongation @ Brk (TD) % 884 600 769 543 (MD) SEC. MOD @ % STRN,% 18311 21257 19583 19174 (TD) SEC. MOD @ % STRN, % 18994 21051 2144319326 % STRAIN, % 2 2 2 2 (MD) TENSILE STR @YlD, psi 1529 1714 1791 1520(TD) TENSILE STR @YlD, psi 5677 1590 1530 1455 Extrusion Parameters:Melt Temperature (° F.) 423 433 428 428 Head Pressure (psi) 3989 41233941 3941 Die Pressure (psi) 2425 2491 2525 2525 Motor Load (amps) 64.170.3 69.2 69.2

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings therein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andsprit of the present invention. Unless otherwise indicated, all numbersexpressing quantities of ingredients, properties, reaction conditions,and so forth, used in the specification and claims are to be understoodas approximations based on the desired properties sought to be obtainedby the present invention, and the error of measurement, etc., and shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Whenever anumerical range with a lower limit and an upper limit is disclosed, andnumber falling within the range is specifically disclose. Moreover, theindefinite articles “a” or “an”, as use in the claims, are definedherein to mean one or more than one of the element that it introduces.

What is claimed is:
 1. A Ziegler-Natta catalyzed ethylene alpha-olefincopolymer, further comprising: a density of between 0.900 g/cc and 0.930g/cc; a melt index ratio (I₂₁/I₂) of between 20 and 35 dg/min; a meltindex (I₂) of between 0.1 and 10 dg/min; and a polydispersity index(Mw/Mn) of between 3.0 and 5.0; wherein the Temperature Raising ElutionFractionation (TREF) fraction distribution of the copolymer has at least15 wt % of TREF fractions below an elution temperature of 35° C.; andwherein the copolymer is produced by reacting ethylene and analpha-olefin comonomer in the presence of a titanium-based Ziegler-Nattacatalyst in a gas-phase process at reaction temperatures in the range ofabout 50° C. to about 100° C.
 2. The copolymer of claim 1, wherein themelting point of the copolymer is in the range of from about 124° C. toabout 127° C. over the density range of 0.900 g/cc to 0.930 g/cc.
 3. Thecopolymer of claim 1, wherein the molecular weight of the copolymer issubstantially constant over the entire TREF fraction distribution. 4.The copolymer of claim 1, wherein the molecular weight (Mw) of thecopolymer satisfies the formula: (Mw of 35° C.)/(Mw of 100° C.) =1.0 to1.5.
 5. The copolymer of claim 1, wherein the copolymer has ahomopolymer content less than 10 wt %.
 6. The copolymer of claim 1,wherein the TREF fraction distribution of the copolymer is substantiallyevenly distributed across the elution temperature range from about 35°C. to about 100° C.
 7. The copolymer of claim 1, wherein the copolymerexhibits a uniform comonomer distribution across its molecular weight.8. The copolymer of claim 1, wherein the melt index ratio of (I₂₁/I₂) isbetween 20 and 30 dg/min.
 9. The copolymer of claim 1, wherein thepolydispersity index is between 3.0 and 4.5.
 10. The copolymer of claim1, wherein the polydispersity index is between 3.0 and 4.0.
 11. Thecopolymer of claim 1, wherein the polydispersity index is between 3.2and 3.8.
 12. The copolymer of claim 1, wherein the polymers have a ratio(Mz/Mw) of z-average molecular weight (Mz) to weight molecular weight(Mw) of greater than 2.5.
 13. The copolymer of claim 1, wherein theratio (Mz/Mw) is between 2.5 and 3.8.
 14. The copolymer of claim 1,wherein the ratio (Mz/Mw) is between 2.5 and 3.5.
 15. The copolymer ofclaim 1, wherein the ratio (Mz/Mw) is between 2.5 and 3.0.
 16. Thecopolymer of claim 1, wherein a blown film with 25 μm thickness preparedby extruding the copolymer has a dart impact resistance of greater than500 g/mil.
 17. The copolymer of claim 1, wherein a blown film with 25 μmthickness prepared by extruding the copolymer has a MD tear strength ofgreater than 450 g/mil.
 18. The copolymer of claim 1, wherein themelting point is in the range of 124° C. to 126° C. over the densityrange of 0.9140 to 0.9250 g/cc.
 19. The copolymer of claim 1, whereinthe copolymer is produced by reacting ethylene and a comonomer in thepresence of a titanium-based Ziegler-Natta catalyst and an alkylaluminumco-catalyst at a temperature in the range of 50° C. to 100° C., anethylene partial pressure of from 10 psi and 350 psi, and a comonomer toethylene ratio of from 0.01 to 0.50.
 20. The copolymer of claim 1,wherein alpha-olefin comonomer is selected from 1-hexene, 1-octene and1-butene.
 21. The copolymer of claim 1, wherein the titanium-basedZiegler-Natta catalyst comprising: a. magnesium; b. a compound havingthe formula R¹ _(m)Si(OR²)_(n), wherein R¹ and R² are C₁-C₂₀ carbonatoms, m=0-3, n=1-4, and m+n=4, and wherein each R¹ and each R² may bethe same or different; c. a compound having the formula R³ _(x)SiX_(y),wherein R³ is C₁-C₂₀ carbon atoms, X is halogen, x =0-3, y=1-4, andx+y=4, and wherein each X and each R³ may be the same or different; d. acompound having the formula MX₄ and M(OR⁴)X₄, wherein M is a titanium,wherein R⁴ is C₁-C₂₀ carbon atoms, X is halogen, and wherein each R⁴ maybe the same or different; e. a substituted aromatic nitrogen compound;and f. an alkyl halide or aromatic halide compound having the formulaR⁵X, wherein R⁵ is an alkyl group containing 3 to 20 carbon atoms or anaromatic group containing 6 to 18 carbon atoms, and X is selected fromchlorine and bromine.